1. Field of the Invention
The present invention relates to a belt device and an image forming apparatus, and more particularly, to a belt control mechanism for controlling a bias applied to a belt.
2. Description of the Related Art
In a typical image forming apparatus, such as a printer, a copier, a facsimile machine, and a printing press, an endless belt is used as a latent-image carrier, a transfer medium, or a conveying member for conveying a recording medium such as a sheet. Such a belt is generally supported by a plurality of rollers, and moves in a predetermined direction in accordance with rotation of any one of the rollers.
Namely, the belt moves by having frictional contact with the rollers. While the belt is moving, it may happen that the belt meanders by the friction in an axial direction of the rollers, a tilt of an axis line, or the like. In this case, the meandering of the belt means that the belt moves by being biased toward either one side of the belt in a width direction parallel to the axial direction of the rollers.
When a full-color image is formed by a color image forming apparatus capable of forming any of a monochrome image and a full-color image selectively, a plurality of different color toner images are transferred onto a transfer belt in a superimposed manner. However, if the transfer belt meanders, each of the toner images cannot be transferred onto a predetermined transfer position on the transfer belt, and thus an error such as a color registration error occurs in a formed image.
To prevent such a meandering of a belt, various methods have been developed. For example, in a conventional technology disclosed in Japanese Patent No. 3523503, a blocking member is integrally fixed to a rear surface of the belt at a position close to an end face of a roller. If the belt meanders, the blocking member collides with the end face of the roller. Therefore, it is possible to prevent the belt from being biased toward either one side in the width direction.
Furthermore, for example, in conventional technologies disclosed in Japanese Patent Application Laid-open No. HO5-204199, Japanese Patent Application Laid-open No. 2004-226746, and Japanese Patent Application Laid-open No. H11-161055, in addition to such a blocking member, a belt is pressed so as to prevent the belt from running on to an outer circumferential surface of a roller.
Moreover, for example, in conventional technologies disclosed in Japanese Patent No. 3223771 and Japanese Patent Application Laid-open No. H05-134486, instead of a blocking member, a flange is provided on both ends of a roller in an axial direction so that a belt is prevented from meandering by the flange.
In this configuration in which the flange is provided on the ends of the roller, although it is possible to avoid an impact caused when the blocking member collides with the end face of the roller, there are some problems as explained below.
If there is a positional tolerance among the rollers supporting the belt or a deviation of the tension applied to the belt in the axial direction of the rollers due to a tilt of an axis line of each of the rollers, an excess bias force in the width direction of the belt occurs. Furthermore, such an excess bias force occurs when each of the rollers differs in a coefficient of friction in the axial direction.
When the belt is excessively biased in the width direction, and struck on the flange, the belt is buckled. If such a state that the belt is buckled is continued for a long time, the durability of the belt is impaired. It is possible to use a high stiffness belt in consideration of the durability of the belt; however, a cost increases.